1. Field of the Invention
The invention relates to adhesive powders and, more specifically, to microfine powders of adhesive blends wherein the particles are spherical or substantially spherical in shape and comprised of polyolefin base resins and acid or acid derivative functionalized polyolefins and to a dispersion process for their preparation.
2. Description of the Prior Art
Adhesive blends, commonly referred to as tie-layer adhesives, designed to improve adhesion between polyolefins and dissimilar substrates, such as polyamides, EVOH copolymers, metals and the like, are well known. These adhesive blends typically contain a polyolefin base resin as the predominant component and a modified polyolefin containing carboxylic acid or anhydride functionality, e.g., polymers grafted with maleic anhydride, as the minor component. Optionally, one or more other polymeric materials, including rubbers, are included in the adhesive blends. Representative adhesive blends of the above types are described in U.S. Pat. Nos. 4,087,587; 4,298,712; 4,487,885; 4,774,144 and 5,367,022.
Adhesive blends of the above types are obtained by melt blending the components and can be used directly in extrusion or coextrusion processes. More typically, however, for commercial applications the melt blends are formed into pellets which can be easily stored and used at a later date by the processor.
Thermoplastic resin powders are widely used in industry for a variety of applications. For example, powdered thermoplastic resins are used to coat articles by dip coating in either a static or fluidized bed or by powder coating wherein the powder is applied by flame or electrostatic spraying or dusting. In recent years increasing emphasis has been placed on the use of powders which chemically bond to the surface of the structures, e.g., metal surfaces, such as aluminum or steel or polymer surfaces, such as nylon, ethylene-vinyl alcohol (EVOH) copolymers or polyolefins. In the case of materials like steel, coating with these powders provides protection against corrosives and other reactive materials.
For certain applications, the shape and size of the powder particles are important considerations. For example, for most effective fluidization and dry spraying it is generally considered advantageous to use powders which have a fairly narrow particle size distribution and wherein the particles are spherical or substantially spherical in shape. Powders produced by mechanical grinding or pulverization typically have particles which are irregular in shape and particle size distributions which are quite broad.
Particle size distribution is determined using U.S. Standard Sieves or light scattering techniques and, depending on the method used, will be reported in mesh size or microns. The inverse relationship between the sieve size (mesh number) and particle size (in microns or micrometers) is well documented and conversion tables are available. The shape of the particles is ascertained from photomicrographs of the powders. Particle shape has a marked influence on the bulk density of the powders and its handling properties.
It would be highly advantageous if microfine adhesive powders which exhibit superior adhesion to a variety of substrates were available. It would be even more advantageous if the microfine powders were comprised of small particles which are spherical or substantially spherical in shape. It would be particularly advantageous if microfine powders having narrow particle size distributions were available. These and other advantages are achieved with the microfine adhesive blend powders produced by the process of the present invention which will be described in more detail to follow.